Wind-powered electrical generators in current use commonly employ a horizontal-axis, propeller-like, wind turbine to capture power from air flowing parallel to the rotational axis of the turbine blades. However, as the wind direction can change, such turbines need to be mounted so that they may pivot about a vertical axis in order that they may face directly into the wind.
Aspects of the present invention, on the other hand, are based on a design of turbine known as a Darrieus wind turbine. In such turbines, the blades rotate about an axis perpendicular to the wind direction, and as such can be driven by wind from any direction, without the need for reorientation.
In 1931, G. J. M. Darrieus disclosed, in U.S. Pat. No. 1,835,018, a three-bladed wind turbine mounted on a vertical rotating shaft. Since that time, the Darrieus turbine has received substantial attention as an effective means of power generation.
Various configurations of generator and blades have been tried, but presented disadvantages in either placing the blades close to the ground, where wind speeds are low, or placing the generator and brakes at some height, making them inaccessible for maintenance purposes. Larger embodiments of the Darrieus design have required the use of guy wires to restrain lateral vibrations and high wind loadings, and these guy ropes occupy significant space around the turbines.
U.S. Pat. No. 4,449,053 discloses a vertical axis wind turbine of the Darrieus type with guy ropes, means for lubricating an upper bearing, and a hinged tower, for assembly on the ground before raising.
U.S. Pat. Nos. 5,375,324 and 5,499,904 disclose similar vertical axis turbines having the disadvantages that the blades extend close to the ground, where wind speeds are lower, and that guy ropes are required for support. They also have the disadvantage that a stationary tubular axle is required that passes through the inside of the rotating tube on which the blades are mounted, and supports it by at least one upper bearing and at least one lower bearing. This stationary axle, by virtue of its diameter which is smaller than that of the rotating tube, necessitates an inefficient usage of material, as its bending stiffness is inherently lower than that of a tube of similar weight and length but with a larger diameter and thinner walls.
U.S. Pat. No. 6,672,837 recognizes the problem of oscillations in wind turbines, and describes a method for damping oscillations in the nacelle of a horizontal axis wind turbine, this being a turbine where the fluid flow is parallel to the rotational axis. This makes use of a damping device that does not rotate with the blades, and is therefore unsuitable for the particular case of a vertical axis wind turbine of the type described below, where the most suitable mounting point for a damping device is on a rotating part of the turbine. It also addresses the oscillatory phenomena associated with blades that rotate about an axis parallel to the fluid flow, whereas the present invention relates to the problems associated with blades rotating about an axis perpendicular to the fluid flow, which include significant excitational forces at a frequency fn, where f is the first order rotational frequency of the turbine, and n is the number of blades. The '837 patent also addresses the case of a generator mounted in a nacelle at the top of a tower, an arrangement inherently more prone to lower frequency resonances, whereas aspects of the present invention relates to turbines with generator and/or other substantial components located at the base, resulting in higher frequencies of natural oscillation.
U.S. Pat. No. 7,692,322 recognizes the problem of oscillations in wind turbines, and describes a method for active damping of oscillations at the nacelle of a horizontal axis wind turbine. This patent has the disadvantage that it requires and depends upon reliable operation of electronically controlled sensors and expensive actuation hardware, which modify the pitch of the blades in order to counteract a detected oscillation.
U.S. patent application Ser. No. 13/304,505 to de Salis et al, which is incorporated herein by reference in its entirety, discloses certain types of blades for a fluid turbine, which may be advantageously used with various embodiments of the present invention.